The Hepadnaviradae family consists of closely related, yet species specific, DNA viruses which replicate via reverse transcription. Studies of human Hepatitis B Virus (HBV) and Woodchuck Hepatitis Virus (WHV) have shown that both viruses are mainly hepatotropic and contain four open reading frames that encode the major viral proteins: Core, Polymerase, Surface and X. The two viruses share approximately 59% nucleotide identity and have similar physical maps (Galibert et al., J. Virol. 41:51-65,1982). Although spliced HBV RNAs have been reported, the major HBV and WHV proteins are translated from unspliced RNAs. The viral RNAs terminate at the same polyadenylation site and share a common 3' termini.
The correlation of HBV infection with an increased risk of hepatocellular carcinoma has stimulated research into virus-host interactions and gene regulation of the Hepadnaviradae. Transcription of the major viral proteins is mediated by four promoters which are partially regulated by HBV enhancers I and II. HBV enhancers I and II have been shown to upregulate heterologous promoters and are believed to be key determinants of HBV hepatotropism. Both enhancer I and II are liver specific, although enhancer I retains low activity levels in some non-hepatic cells. HBV enhancer I maps upstream of the X open reading frame and consists of a modulatory domain, a core enhancer domain and a basal X promoter domain. Enhancer II maps to Core promoter region and is thought to influence levels of genomic RNA.
The transcriptional regulatory elements of WHV are not well characterized. Mapping studies have confirmed that WHV contains promoters analogous to the major HBV promoters (Di et al., Virology, 229:25-35, 1997; Sugata et al., Virology 205:314-320, 1994). Recent studies have shown that WHV enhancer II is a strongly liver specific enhancer that regulates the production of pregenomic RNAs, which is an important rate limiting step of hepadnaviral replication. Surprisingly, the WHV region homologous to HBV enhancer I lacks enhancer activity in the three human liver cell lines tested. This region failed to activate transcription of the four viral promoters and did not effect a heterologous thymidine kinase promoter. The authors suggest that either the human liver cells do not express the required transcription factors or that major differences exist in the transcriptional control of HBV and WHV.
The HBV Posttrancriptional Regulatory Element (HPRE) is an orientation dependent cis-acting RNA element that partially overlaps with Enhancer I and is required for the cytoplasmic localization of HBV Surface RNAs. The HPRE does not require a virally encoded protein and is believed to interact with cellular proteins which mediate export of the intronless Surface RNA. The HPRE can finctionally substitute for the HIV-1 Rev/Rev Responsive Element (RRE) complex in a transient transfection reporter assay.
In most cases, cellular mRNAs contain introns that are removed by splicing before transport to the cytoplasm occurs. Transport to the cytoplasm is required for the mRNA to interact with the ribosomes and accessory factors in the process of protein synthesis. Recent studies have suggested that intron-containing RNAs are usually prevented from exiting the nucleus due to the binding of splicing factors (Chang and Sharp, Cell 59:789-795, 1989; Legrain and Rosbash, Cell 57:573-583, 19989); although there are a few examples of differentially spliced cellular transcripts that are transported with a retained intron. Little is know about the mechanisms that allow these mRNAs to be transported.
The first identified and best characterized viral export system is the HIV-1 Rev/RRE complex (U.S. Pat. No. 5,585,263). HIV-1 Rev has been shown to directly mediate RNA export via its nuclear export signal (NES). A number of simple retroviruses, such as the Mason Pfizer Monkey Virus (MPMV), also encode cis-acting RNA export elements. MPMV encodes a structured RNA element required for the export of the intron-containing genomic RNA. An additional element has been found in the thymidine kinase (tk) gene of the Herpes Simplex Virus-1 (HSV-1). It has also been reported that hnRNP L binds to a site within the tk gene, and using mutants of the tk gene, showed a correlation between hnRNP L binding and RNA export. All of the cis-acting elements are essential for the cytoplasmic localization of viral RNA and, with the exception of the complex retroviral elements, are thought to interact with cellular RNA export proteins.
High levels of transgene expression are desired in most protocols of gene therapy. Gene delivery systems utilized for this purpose include retroviral vectors, adenoviral vectors, vectors derived from the adeno-associated virus and from herpes virus, as well as non-viral vectors. Retroviral vectors, in particular, can only transfer sequences as cDNAs instead of complete intron-containing genes, because efficient introns are spliced out during the sequence of events leading to the formation of the retroviral particle. Introns mediate the interaction of primary transcripts with the splicing machinery. Because the processing of RNAs by the splicing machinery facilitates their cytoplasmic export, due to a coupling between the splicing and transport machineries, cDNAs are often inefficiently expressed.